WAYS OF THE WILD.

Auckland Star, Volume LXVI, Issue 243, 14 October 1933, Page 1 (Supplement)

WAYS OF THE WILD.

SENSE ORGANS OF FISHES. SIGHT AND SMEUU

(By A. T. PYCROFT.)

A correspondent, having read a recent article in this column, "Can Fishes Hear," wishes to know if fishes have keen sight, or are they guided by smell. There is no doubt that the sense of sight in some fishes is keen; it is well known that a trout will show a preference for a particular kind of fly, whether natural or artificial, and will seize it with great rapidity when it is placed within its range of vision. A trout has been known to refuse a certain kind of fly again and again, but when another was substituted of similar size and shape, but of different colour, it was promptly accepted. This would suggest that the trout is sensitive to colour, but whether this is the general rule among fishes is not clear. Experiments have shown that the sense of sight probably plays the most important part in the search for food, but, at the same time, this is much more limited than that of a land vertebrate, and, owing to the general haziness of the water, due to the presence of organisms and other matter suspended therein, objects must appear of somewhat uncertain outline. The extreme convexity of the lens of the eye points to the fact that a fish is nearsighter, and, even in the clearest water, it is doubtful whether the range of vision exceeds about twelve yards, if as far as this. It is not unlikely that the fish really notices movements or changes in outline rather than actual objects. In its general form the eye of a fish is not unlike our own, but it is necessarily somewhat modified for vision under water. The eye, as is well known, acta after the manner of a photographic camera, the two essential parts being the sensitive screen or retina at the back and the lens at the front, which projects an image of the outside world on. the screen. The lens of a land vertebrate is somewhat flat and convex on both sides, but in the fish it is a globular body, the extreme convexity being a necessity under water, because the sub-, stance of the lens is not very much denser than tho fluid medium in which the fish lives. Fishes possess no true eyelids, the skin and integuments of the head simply passing over the eye and becoming transparent as they cross the orbit. Some sharks have a third eyelid known as the nictitating membrane at the front corner of the eye, which is fully movable and can be pulled down to cover the whole surface.

Curious Telescopic Eyes. Some oceanic fishes are provided with curious telescopic eyes, and these generally take the form of short, protruding cylinders each ending in a very rounded .cornea. They may be directed either upwards or forwards, and as they lie parallel tp one another it is possible that these fishes are capable of binocular vision. In one rare and curious oceanic fish the telescopic eyes are directed upwards, and cannot be turned in any other direction. In the young of other oceanic forms the eyes are placed at the ends of very long stalks growing out from the sides of the head. The eyes of the hammer-headed shark are placed at extremities, A curious,,modification of the eyes is found in the four-eyed fishes of the rivers of Central and South America. Each eye projects well above the top of the head, and is divided into two equal parts by a dark horizontal band; each of these sections is of a different structure, the upper being adapted for vision in the air, the lower for vision under water. These fishes swim about in small 6lioals at the surface of the water, and the level of the water reaches as far as the bar dividing the eyes. Thus they are enabled to detect not only insects skimming over the surface, or actually flying in the air, but also any swimming below the surface. The position of the eyes departs from the normal in some fishes, and in those living at the bottom, such as the rays, anglers and star-gazers, instead of being placed on either side of the head the two eyes lie close together on its upper surface. The flat fishes are unique in having both the eyes on the same 6ide of the head. In many fishes living on or near to the sea floor in the abyssmal depths, the eyes are comparatively large and well developed, but luminous organs are absent or but feebly developed. This curious fact can only be explained on the assumption that these oceanic abysses are not completely dark, and it is probable that the invertebrate bottom animals, which are known to be luminous, emit light of sufficient strength to make objects on the bottom visible to these fishes. Sense of Smell in Fishes. There can be little doubt that the sense of smell in fishes is relatively acute, as has been proved by numerous experiments. The large nasal organs of sharks are said to enable them to "scent actively as well as to smell passively," and it is well known that the smell of flesh or blood, or of a decaying carcase will attract them to it from some distance away. The caribe, or piraya, a ferocious fish of the rivers of South America, is irresistibly attracted by the smell of blood, and woe betide the animal unfortunate enough to be bitten by one of these pests, for hundreds more will rush to the spot with incredible rapidity. As long ago as 10-53 Izaak Walton wrote the following in his "Compleat Angler" with reference to the sense of smell in fishes: "And now I shall tell you that which may be called a secret. I have been a-fishing with old Oliver Henly, now with God, a noted fisher for trout and salmon, and have observed that he would usually take three or four worms out of his bag, and put them into a little box in his pocket, where he would usually let them continue half an hour or more before he would bait his hook with them. I have asked him his reason, and he has replied that he did but pick the best out to be in readiness against he baited his hook the next time; but he has been observed, both by others and myself, to catch more fish than I or any other body that has ever gone a-fishing with him, could do, and especially salmons. And I have been told lately, by one of his most intimate and secret friends, that the box in which he puts these worms was anointed with a drop, or two or three, of the oil of ivy berries, made by expression or infusion, and told that by the worms remaining in that box an hour, or a like time, they had incorporated a kind of smell that was irresistibly attractive enongh to force any fish within the smell of them to bite."